Polymer fibres with diameters between 10 nm to 1,000 nm represent a new grade of materials with some very valuable properties. Such a typical field of use of polymer fibres layers is a filtration of gases and liquids, barrier materials for entrapment of submicron particles, bacteria and chemicals, where there is a very high filtering efficiency reached. Nanofibres are used as battery separators, composite reinforcement and as pharmaceutical carriers and tissue implant carriers in medicine. The high specific surface of the nanofibres makes them easily accessible to gaseous and liquid media, gives them their special sorptive properties and makes them suitable for their use as carriers of different active ingredients, e.g. catalysators. Extremely small pores in layers of nanofibres are a condition for extreme thermal insulating properties.
Nanofibres are made of a broad range of polymers, polymer blends and from blends of polymers with low molecular additives by forming processes involving polymer solutions. Unlike similar processes of forming fibres from polymer melts, forming fibres by processing polymer solutions can produce fibres with smaller diameters due to lower viscosities of the polymer solutions. For forming fibres from polymer solutions, mechanical forces of a flowing gaseous medium or coulombic forces in an electrostatic field can be used. Electrostatic spinning leads to fibres of lower diameters because a single fibre will split into a number of filaments owing to the distribution of equivalent charge in their volume.
Conventional methods and devices for production of nanofibres by polymer solutions forming by an air stream are described for example in U.S. Pat. No. 6,382,526 and U.S. Pat. No. 6,520,425. Polymer solutions are injected into a spinning jet of an annular section. The solutions are then formed by a mechanical action of an air stream delivered inside of the annulus, or as the case may be outside of this annulus, to produce fibres of diameters of 200 nm to 3,000 nm.
Forming of polymer solutions using an electrostatic field of mean intensity 50,000 V/m to 500,000 V/m is described in patent applications WO 0.127.365, WO 0.250.346, US 2002/0.175.449 A1 and US 2002/084.178 A1. According to these methods, the polymer solution is distributed into cylindrical spinning jets with inside diameters 0.5 mm to 1.5 mm. These jets are connected to a source of DC voltage. The electrostatic force attracts the effluent solvent to the counter electrode, which is usually grounded, and at the same time the effluent solvent is by this force formed into fine filaments, which are consequently split in a filament bundle of corresponding smaller diameter. Spinning is performed from one jet or an array of static or moving jets with the aim to increase the capacity of the device, even out the coverage of the counter electrode or the planar supporting material moving on a surface of the counter electrode or in the vicinity of its surface.
The drawback of all above mentioned methods and devices for production of nanofibres is that a very small amount of polymer material can be processed in a given time. In the case of nanofibres formed by mechanical forces, the diameter of the nanofibres so produced depends on, among other things, a ratio of air mass and polymer solution mass flowing through the spinning jet. While forming nanofibres by coulombic force in an electrostatic field, there must be formed a so called Taylor cone at the throat of the spinning jet, whose existence is a requirement for fibres formation, and the formation of the Taylor cone requires a relatively narrow range of the ratio of discharge velocity of the polymer solvent from the spinning jet to the intensity of the electrostatic field. The maximum adjustable intensity of the electrostatic field is limited by the dielectric strength of air, and above this limit discharges between electrodes happen. In consequence of the above mentioned circumstances and attainable concentrations of spinning polymer solutions, it is possible to process approximately 0.1 g to 1 g of polymer in an hour in one spinning jet, which from the industrial point of view makes the production of nanofibres very problematic.
The aim of the invention is to create a method and a device industrially applicable and able to reach a high spinning capacity.